The constantly increasing demand for higher data rates in cellular networks requires new approaches to meet the demand. Different mechanisms have evolved for increasing the data rates of cellular networks such as increasing the density of the macro base stations (BS), increasing the cooperation between the macro base stations and deploying smaller base stations or relay nodes (RN) in areas where high data rates are required within the macro base station grid. The option of deploying smaller base stations or relay nodes in the macro base station grid is generally referred to as a heterogeneous deployment (creating a heterogeneous network) and the layer of smaller base stations is known as a micro-layer or pico-layer depending on the characteristics of the smaller base stations.
Although each of the above-described choices would result in increasing the data rates of a cellular network, economics associated with those choices typically dictate that creating a heterogeneous network would be the most cost-effective implementation. Further, the implementation time frames requested by operators also seems to favor heterogeneous network solutions. As an example of heterogeneous deployment, and looking to FIGS. 1a and 1b, a homogeneous cellular network 100 can be illustrated as a collection of cells 102, 104, 106, 108, 110, each of which represent the radio communication coverage area of a macro base station. FIG. 1b illustrates an exemplary heterogeneous network where cells 102, 104, 106, 108, 110 still provide radio communication coverage via their respective macro base stations, but where that coverage is augmented by the provision of micro/pico base stations 112, 114, 116 within the cell areas of macro base stations 102, 104, 110, respectively, by way of a heterogeneous deployment.
One of the objectives of creating heterogeneous networks is to allow the micro/pico base stations to offload as many users as possible from the macro layer, allowing higher data rates in both the macro layer and the micro/pico layer. To this end, different techniques have been proposed for increasing the capacity of the micro/pico base stations. First, capacity can be increased by extending the range of the micro/pico base stations using cell specific cell selection offsets. Cell selection offsets are one factor used to determine whether a user equipment should connect to the heterogeneous network via a micro/pico base station or a macro base station. Second, capacity can be increased by simultaneously increasing the transmission power of the micro/pico base stations and appropriately setting the uplink (UL) power control target (P0) for the users connected to the micro/pico base stations.
Under certain circumstances, e.g., prohibitive backhaul costs associated with adding a micro/pico base station, a relay node (RN) can provide a viable solution to provide increased range and/or capacity based its usage of an in-band (wireless) backhaul. The relay node can provide pico base station type coverage either indoors or outdoors and mitigate the cost and effort of deploying land-line backhaul to all of the pico base stations. In a further scenario, there are users on mobile platforms, i.e., commuter/passenger trains that would benefit from a mobile relay node. The implementation of a mobile relay node involves local access from the mobile relay node to the users on the mobile platform and in-band backhaul bandwidth from the mobile relay node to a stationary serving macro base station or an eNB.
A problem identified with heterogeneous networks employing relay nodes is that the backhaul link (Un) between the serving or donor base station and the relay node can generate additional interference, above normally expected levels, in the macro network. The increased interference can reduce the capacity of the macro network, therefore undermining the intent of creating the heterogeneous network. For example, as depicted in FIG. 2a, Un uplink transmissions 208 to a given macro base station 204 from a relay node 210 can cause interference 212 in the backhaul Un uplink transmissions 214 of relay nodes 216 in adjacent macro base stations 202. Furthermore, the Un uplink transmissions 208 from relay nodes 210 within one macro base station cell 218 can interfere 220 with uplink transmissions between the terminals or user equipment (UE) 222 to their serving relay nodes 224 in neighboring macro base station cells 226.
A reciprocal problem can occur wherein the downlink (DL) transmission on the Uu link can cause interference in the downlink Un link of neighboring cell relay nodes. It should be noted that these scenarios are likely to occur because typical deployments for relay nodes are those in which the relay nodes are placed at cell edges of neighboring donor macro base stations, thus resulting in the placement of relay nodes supporting adjacent macro base stations in close proximity to each other. Considering mobile relay nodes, the potential interference scenarios are further exacerbated when the mobile relay node moves closer to the serving eNB of the donor macro cell. In this mobile relay node scenario, the user equipment associated with the donor macro cell that are near the edge of the donor macro cell can be severely interfered with by the backhaul Un link of the mobile relay node to the donor eNB. Furthermore, if the mobile relay node gets too close to the donor eNB it could completely desensitize the front end of the donor eNB and cause an outage to all of the users served by the donor eNB.
Considering LTE networks, the existing approach to mitigating this type of interference involves time multiplexing of the Un and Uu transmissions within a donor macro cell to reduce the potential Un to Uu interference. The two main issues with this type of interference mitigation approach are first, the time multiplexing reduces the interference within a given donor macro cell but it does not guarantee reduction of interference between relay nodes of adjacent macro donor cells, with the problem being aggravated by the mobility of the relay node and second, even though the relay node can use directive antennas for the Un link, the side lobes and/or back lobe of the relay node antenna for the Un link can still cause significant interference to a relay node's Uu link in neighboring macro donor cells. This latter issue is most apparent when a mobile relay node is in close proximity to the donor eNB or a remote radio head (RRH) of the serving macro donor cell.
It should be noted that the above described situation in an LTE network can occur when mobile relay nodes are deployed near the edge of neighboring macro donor cells, which, as described above, is the most like position for deployment of mobile relay nodes. Although in theory, restrictions in the time domain regarding when neighboring macro donor cell's relay nodes can transmit on their Un and Uu links might be sufficient to mitigate interference, this would require strict time synchronization between neighboring macro donor cells and the mobile relay nodes within the neighboring donor macro cells and in general, cellular networks may not be time synchronized.
Accordingly, efforts for a method of reducing interference in unsynchronized cellular networks deploying mobile relay nodes are of importance to service providers and indirectly to the customers accessing the cellular network.